Biological mechanisms in vivo are extremely complicated cascades of signals, which are difficult to deconvolute and understand. An example of such signalling is that required to activate B-cells. The B cell antigen receptor (BCR) is composed of membrane immunoglobulin (mlg) molecules and associated Igα/Igβ (CD79a/CD79b) heterodimers (α/β). The mIg subunits bind antigen, resulting in receptor aggregation, while the α/β subunits transduce signals to the cell interior. BCR aggregation rapidly activates the Src family kinases Lyn, Blk, and Fyn as well as the Syk and Btk tyrosine kinases. This initiates the formation of a ‘signalosome’ composed of the BCR, the aforementioned tyrosine kinases, adaptor proteins such as CD19 and BLNK, and signaling enzymes such as PLCγ2, PI3K, and Vav.
Signals emanating from the signalosome activate multiple signaling cascades that involve kinases, GTPases, and transcription factors. This results in changes in cell metabolism, gene expression, and cytoskeletal organization. The complexity of BCR signaling permits many distinct outcomes, including survival, tolerance (anergy) or apoptosis, proliferation, and differentiation into antibody-producing cells or memory B cells. The outcome of the response is determined by the maturation state of the cell, the nature of the antigen, the magnitude and duration of BCR signaling, and signals from other receptors such as CD40, the IL-21 receptor, and BAFF-R.
Many other transmembrane proteins, some of which are receptors, modulate specific elements of BCR signaling. A few of these, including CD45, CD19, CD22, PIR-B, and FcγRIIB1 (CD32). The magnitude and duration of BCR signaling are limited by negative feedback loops including those involving the Lyn/CD22/SHP-1 pathway, the Cbp/Csk pathway, SHIP, Cbl, Dok-1, Dok-3, FcγRIIB1, PIR-B, and internalization of the BCR. In vivo, B cells are often activated by antigen-presenting cells that capture antigens and display them on their cell surface. Activation of B cells by such membrane-associated antigens requires BCR-induced cytoskeletal reorganization.
Autoreactive B cells are responsible for the production of pathogenic autoantibodies which can either directly or indirectly cause or exacerbate autoimmune conditions. Depletion of CD20 positive B cells has been used to successfully treat a number of autoimmune conditions and thus established conclusively that B cells play an important role in causing or maintaining a number of autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis and type I diabetes mellitus. Although B cell depletion has been a successful therapeutic option evidence also exists that control of B cell growth and activation status can also be an effective way to modulate B cell function. Alternative strategies that do not deplete B cells and offer the flexibility of controlling B cells without long term suppression of B cell immunity, which has been shown to be associated with some side effects would therefore be desirable. In addition not all B cell responses or activities are harmful and evidence suggests that maintenance of regulatory B cell populations can be protective. Such an approach should be effective in diseases which have abnormal B cell function caused by inappropriate or excessive BcR signalling. Examples of such diseases include, but are not limited to, inflammation, autoimmunity and cancer. Of particular interest are diseases that either have a direct requirement for BcR signalling or require inhibition or stimulation of humoral immune responses.
Co-ligation of Fc gamma receptor IIb (CD32b) with the B cell receptor occurs to naturally regulate signalling, in particular when antigen is bound to antibody in small immune complexes. CD32b then recruits the phophatases SHP-1 and SHIP-1 which antagonise BcR activation. Although this natural regulatory mechanism can control B cell function, disruption of CD32b function caused by variation in the protein sequence of CD32b can lead to autoimmune disease and this receptor can be down regulated in autoimmune disease—e.g. as in the case of SLE. Alternative ways of blocking B cell activity are thus desirable as they offer alternative, non-natural, ways of regulating BcR function. These alternative mechanisms are likely to be particularly important when natural mechanisms are dis-functional in the given disease.
CD22 is an inhibitory coreceptor of the B-cell receptor (BCR) and plays a critical role in establishing signalling thresholds for B cell activation. Targeting CD22 induces both physical down-regulation of BCR complex components on resting cells, which would likely result in less responsive B cells as well as limiting the extent of BCR signalling after BCR engagement. The overall effect is inhibition of B-cell activation, ultimately reducing subsequent autoimmune and inflammatory events mediated by B cells. Therefore modulation of CD22 function by antibodies can have therapeutic benefit in many diseases dependent upon B cell activation including autoimmuninty, immunodeficiency, and malignancy, (see for example, CD22: an inhibitory enigma. Immunology. 2007. Vol 123: 314-325). The present disclosure provides a number of antibody molecules specific to CD22, which may be employed alone or in combination with an entity, such as an antibody or binding fragment thereof specific to a further B cell surface receptor (such as CD79), useful in controlling aberrant B cell functions, for example associated with certain diseases such as autoimmunity and cancer.